The applicant and associates recently described and demonstrated a new modulation system, termed a TDHM (i.e., Time Domain Holography Modulation), refer (1) Characteristics of and Requirements for Subnanometer-Wavelength Multiplexing, Technical Digest of the 1981 International Conference on Integrated Optics and Optical Fiber Communication (IOOC '81), Apr. 27-29, 1981, San Francisco, CA, by C. S. Ih and Bing-Yan Chen; (2) A Direct RF Compatible Optical Fiber Transmission System, Proceedings of the International Conference on LASER '80, Dec. 15-19, 1980, New Orleans, LA by C. S. Ih, Bing-Yan Chen and A. Mallya; (3) Subnanometer-Wavelength Multiplexing for Fiber-Optical Communication, Journal of the Optical Society of America (JOSA), 70, 1569 (December 1980) by C. S. Ih and Bing-Yan Chen; (4) C. S. Ih, JOSA, 68, 1384 (October, 1978); and (5) U.S. Pat. No. 4,210,803 issued July 1, 1980 to Charles S. Ih.
Basically, TDHM can be achieved with a single frequency laser and with apparatus similar to a Mach-Zender interferometer, such as described by W. E. Martin in his article (6) entitled A New Waveguide Switch/Modulator for Integrated Optics, Appl. Phys. Lett., 26, 526 (May 1975), modified by the inclusion of an AO (Acousto Optical) or SAW (Surface Acoustic Wave) modulator.
However, by incorporating an AO or SAW modulator in one of the arms, the modulated laser beam is automatically put on a (sub) carrier. The laser beam can then be effectively Amplitude, Frequency, Phase or Single Side Band (SSB) modulated (with respect to the subcarrier) through the input of the Acoustic Optical Modulation (AOM) as described in references (4) and (5) supra.
By using frequency up-shifting and down-shifting alternatively, it appears that high orders (up to the third) fiber dispersions (for single mode fibers) can be compensated (7) C. S. Ih, Feasibility and Requirements for Dispersion Compensation in Coherent FOC, submitted to the International Symposium on Optical Waveguide Sciences, June 20-23, 1983, Guilin, China.
As described in the Ih patent, reference (5) supra, in TDHM the laser beam is first split into two beams, one of which is reserved as a reference beam. The other beam is the information beam and is modulated and frequency-shifted by an acousto-optical (AO) or surface-acoustic wave (SAW) modulator. SAWs are used for implementing TDHM in an integrated-optics (10) form. The information and reference beams are then combined and coupled into an optical fiber for transmission. A photo diode, which is a square-law detector, detects the beat frequency between the reference and information beams and thus reproduces the original signal (on a carrier) at the receiving end, all as detailed in reference (5) supra. The carrier F.sub.c can be a frequency anywhere in the range from several tens of mHz to tens of GHz.
The TDHM system can be used for subnanometer wavelength multiplexing (refer references (1) and (3) supra) and for direct RF compatible transmission (refer reference (2) supra). Despite these advantages, because the Bragg angle conditions must be satisfied for the AO and SAW modulators, and also because of their finite transition time, the information bandwidth for each channel is limited. Large information bandwidth can be attained only by using a large number of channels. In order to minimize cross-modulation between channels, a strong reference beam must be used. Optical power channeled into the reference beam is largely wasted, since it carries no information. For these reasons, the TDHM per se is not well-suited for either long distance digital transmission or for high bit rate.
This invention provides an improved TDHM whereby both digital (and analog) information can be efficiently transmitted. Since the digital (and some analog) information is automatically put on a "carrier", the new system is suitable for increasing digital rate through Frequency Division Multiplexing (FDM). The potential advantages of my FDM system over the conventional TDHM system are as follows: (a) the noise is reduced due to the reduction of the effective bandwidth (for each channel), (b) the capacitance of the photo diode can be "neutralized" with inductors or can be made part of a resonant cavity, since the information is modulated on a carrier with a "moderate bandwidth", (c) all of the electronic circuits and lasers are operated in parallel at lower speeds as compared with high speed serial operation, and (d) a further reduction in noise may be possible by using a new optoelectronic heterodyne technique (8), R. I. MacDonald and K. O. Hill, Avalanche Optoelectronic Downconverter, Optics Letters, 7, 83 (February 1982.) In at least some instances, the low speed parallel-operated FDM channels are more economical and more reliable than the high speed serially operated TDM (Time-Division-Multiplexing) system. Also, one of the carriers can be utilized as a clock for digital reception or for synchronized detection.